Most wave editor software is capable of doing this. I would recommand cooledit, allthough there will be others coming up with other editors..
I did not enhance voices though as I do use external gear to do this., but I know it can be done by software.
Just load the wave or aif file and you will enjoy the possible and weird effects...
Probably there are some tips about your specific question on the website of Syntrillium as there was a user group I can remember...I haven't been there a long time ,
so I don't have the link, but any search on the web will find it..
Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76